Early in the television era, testing indicated that vertical polarization of broadcast signals was inferior to horizontal in view of the relative immunity of the latter to multipath degradation in urban canyons. This judgment drove broadcast design for some decades. Recently, digital television (DTV) has been chosen as a replacement for traditional analog television (ATV), adding the possibility of higher resolution and reduced noise thanks to the capability of digital signal processing to overcome transmission limitations of ATV bandwidth and propagation.
Initially, it was observed that circular polarization (CP) added no benefit to DTV transmission—indeed, it was determined that CP was detrimental to eight-level vestigial sideband modulation (8-VSB) signals, selected by the Advanced Television Systems Committee (ATSC) for U.S. broadcasting, since CP appeared to be intolerant of multipath. In particular, the vertical component within a circularly polarized signal is intrinsically more susceptible than the horizontal component to multipath distortion, so that use of CP is likely to render DTV signals unrecoverable—even moreso than CP used for the more accommodating ATV signals. This led to selection of horizontal polarization (HP) as the standard for DTV.
Subsequent advances in echo canceling, however, have made 8-VSB much more tolerant of multipath, so that other forms of transmission, including circular polarization, are now feasible. At the same time, U.S. broadcasters have been enabled to add mobile-receiver television service to the previously enabled fixed-receiver service within digital channels already licensed. This new technology (In-Band Mobile TV), may empower broadcasters to establish a mobility advantage over cable competition, and to compete against recently initiated 700 MHz mobile services.
In achieving reliable service in a mobile application, elliptical polarization has distinct advantages over HP. Elliptical polarization (the limit is CP, where the vertical and horizontal components are equal in magnitude) allows receiving antenna orientation and change of orientation to be substantially unimportant to successful broadcast reception. Lower vertical component energy is still desirable—that is, ellipticity is preferably not at a value of one. However, known techniques for distribution of power to elliptically polarized, high power, single-feed broadcast antennas intrinsically provide substantially equal power in vertical and horizontal components, or require unequal power splitters—typically one per radiator—to adjust component energy. Similar results can be achieved in the alternative with dual-feed antennas, incurring instead penalties of higher wind loading, weight, and/or material cost associated with remote power splitters or like solutions. What is needed is a way to adjust relative signal strength between the two component parts of an elliptically polarized signal that is highly efficient, and that still permits the use of preferred styles of broadcast radiators.